Hong Kong lawmakers are expected to vote as early as Thursday on a contentious electoral reform bill backed by Beijing but rejected by the territory's pro-democracy groups.

The reform package, formulated by Beijing, lays out the rules for electing the territory's chief executive in 2017. It would for the first time allow Hong Kongers to vote for their top leader, but candidates would need to be approved by a 1,200-member committee dominated by Beijing loyalists.

China has argued the new proposals are more democratic than anything that existed under more than a century of British colonial rule that ended in 1997.

Pro-democracy activists - many of whom mounted large-scale protests against the measures last year - say Beijing's proposed vetting process is not democratic enough.

Some critics have voiced fears that the measures could result in a repeat of 2012, when the widely unpopular Beijing-backed Leung Chun-ying became the territory's chief executive.

Hundreds of protesters from both sides gathered outside the legislature Wednesday, with government supporters waving Chinese flags and opposition activists decrying the plan they say would lead to "fake democracy."

"There is no such thing as real or fake universal suffrage. We support the National People's Congress Standing Committee's decision," said 65-year-old Beijing supporter Yu Poon-kin.

"I know that this government proposal is a lie," countered pro-democracy protester Brandy Yau. "There is no chance that we will be able to vote for who we really want for our chief executive."

Security was heavier than usual in the Chinese territory after police this week seized a stash of chemicals and arrested 10 people, six of whom have been charged with conspiracy to cause an explosion.

Officials have said little about the alleged bomb plot, which some opposition activists suspect is an attempt to smear a pro-democracy movement that has been overwhelmingly peaceful.